The present invention relates to xe2x80x9cactive implantable medical devices,xe2x80x9d as defined the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, and more particularly to pacemaker devices (for the treatment of bradycardia), defibrillators and/or cardiovertors (for the treatment of tachycardia), as well as resynchronizing xe2x80x9cmultisitexe2x80x9d devices (for the treatment of desynchronization between cardiac cavities).
The devices for which the present invention is useful include a circuit or means for the detection of cardiac activity, i.e., detection of a spontaneous depolarization of the myocardium, and a circuit or means for effecting the stimulation of the myocardium. During and after each activation of the stimulation circuit, which may be detected by monitoring the stimulation circuit or by sensing the myocardium potential, it is envisaged to provide a period known as the xe2x80x9crefractory periodxe2x80x9d, during which there is a disconnection or xe2x80x9cblankingxe2x80x9d of the detection circuits. This is done so as to mask any disturbances of the amplifiers in the detection circuit immediately following the delivery of a stimulation to the myocardium. Such disturbances are, for example, due to the flow of the electrical charges at the electrode/myocardium interface. Typically, the stimulation event is detected by monitoring the operation of the stimulation circuit, knowing that an escape interval has expired and noting that the stimulation capacitor discharges. This also permits applying the refractory period to prevent saturation of the detection circuit amplifiers based on the stimulation pulse.
A long refractory period makes it possible to obtain a good safety margin for the elimination (i.e., non-detection) of these disturbances. However, it also presents the disadvantage of masking any detection of the signals emitted by the myocardium for a relatively long period of time.
It is important to be able to listen to the spontaneous rate/rhythm of the patient as soon as possible after a stimulation, in order to detect most precociously, i.e., as soon as possible, a possible depolarization wave revealing a spontaneous activity of the myocardial cells. This early listening makes it possible to carry out, for example, very precise control algorithms for controlling the rate of the heartbeat, thus giving rise to a more physiological behavior of the implanted medical device (also called a xe2x80x9cprosthesisxe2x80x9d) or to allow a reduction of energy consumption by delivering only suitable stimulation.
This early detection also is used to control advantageously the operation of certain other known algorithms used in such active implantable medical devices, such as the algorithms of fallback of the pacing rate (also referred to as xe2x80x9creplixe2x80x9d), rate smoothing, etc. In addition, the detection of the spontaneous ventricular rhythm, in particular the analysis of its stability, is in certain implantable defibrillators an essential parameter in the release of a shock therapy.
To take account of the various possible situations, the refractory period is generally made variable, with a fixed or programmed portion known as the xe2x80x9cabsolute refractory periodxe2x80x9d (ARP), and a variable portion known as the xe2x80x9crelative refractory periodxe2x80x9d (RRP). One can, for example, vary the RRP according to the programmed stimulation amplitude. This is because the disturbance which one wants not to respond to is shorter in time in the case of a stimulation with a low energy level (for example 1.5 V) than in the case of a stimulation with a higher energy (for example 5.0 V).
Other techniques, for example, the technique referred to in U.S. Pat. No. 4,974,589 and the technique used in the CHORUS brand pacemakers available from ELA Mxc3xa9dical, Montrouge, France, the assignee of the present invention, involve detecting in the course of an ARP a presence of a residual potential at the output of the primary amplifiers of the detection circuits of the endocardium cardiac signal. If a residual potential is detected, one then prolongs the ARP by one predetermined duration, hereafter indicated as an xe2x80x9celementary periodxe2x80x9d, and this elementary period is repeated (restarted or xe2x80x9crecycledxe2x80x9d or xe2x80x9cretriggeredxe2x80x9d) as long as a post-stimulation residual potential appears at output of the detection circuit amplifiers.
When the detected residual potential is lower than a given threshold, the automatic recycling of the elementary period ceases, and at the end of the period the system then passes to a mode of listening for the cardiac signals. In other words, the blanking provided by the refractory period is ended.
It is an object of the present invention to provide an improvement to the active implantable medical devices of the aforementioned type, an improvement which allows for a very fine management of the duration of the relative refractory period RRP so as to reduce the RRP to a minimum time, and thus maximize the duration of listening for the spontaneous rhythm of the patient. It is a further object to implement such an operation to carry out in an optimal way the various control algorithms of the device.
A representative device for use with the present invention is of the type which is described in U.S. Pat. No. 4,974,589. This device is provided with a refractory period applied to the detection circuits after activation of the stimulation circuit. The refractory period includes a fixed or programmed absolute refractory period (ARP), and a variable, relative refractory period (RRP). The relative refractory period includes a succession of elementary periods of fixed or programmable duration which are repeated or retriggered, in response to a detected occurrence of a residual potential of a level higher than a given threshold at an output of the detection circuit, until one elementary period occurs without a detected residual potential occurring.
According to one aspect of the invention, the refractory period is provided with a relative refractory period such that on each retriggering of an elementary period of the relative refractory period, a succession of sub-periods of a given duration are applied to correspond to the elementary period, such that each sub-period is shorter than that one elementary period. More preferably, during each sub-period, the possibility of an occurrence of a residual potential is examined. In the event of a detected residual potential event, the elementary period is then retriggered to begin again as of the end of the sub-period during which the residual potential event was detected. In the absence of a detected residual potential, the sub-period will continue to its end and then be retriggered, if necessary, until the end of the elementary period (that is until all of the sub-periods have occurred) at which time the refractory period will be over.
The duration of the sub-period is advantageously selected to be a submultiple of the duration of the elementary period. In one preferred embodiment, the ratio between the duration of the sub-period and that of the elementary period is at least 2:1, more preferably at least 6:1. However, the sub-periods may be non-uniform in duration.